Nanobubbles (NBs) are gas-filled cavities in liquids with diameters smaller than 1 µm. When the bubble size is less than 1 µm, the buoyancy effect on bubbles is insignificant compared to Brownian motion. As a result, NBs are remarkably stable compared to macrobubbles, and have been observed to remain suspended in liquids for as long as several days. Due to their unique properties, NBs are extremely useful in a broad range of applications, such as froth flotation of fine/ultrafine mineral particles, wastewater treatment, detergent-free cleaning of clothes, and deinking of recycled paper. The coalescence of two NBs is a process of great importance to many of these applications. Therefore, a fundamental understanding of coalescence characteristics of NBs is required for efficient use of NBs in various industrial applications.Despite the importance of NB coalescence in many applications, the characteristics of NB coalescence processes are not fully understood. This is mainly due to lack of experimental tools that can image the ns-timescale coalescence dynamics of NBs with nanoscale spatial resolution. As a result, most experimental studies on characteristics of bubble coalescence focus on the coalescence dynamics of millibubbles and microbubbles.&nbsp;It is not clear if the coalescence characteristics of NBs are similar to those of millibubbles and microbubbles.In this work, we study the coalescence of two equal-sized nitrogen NBs in water using MD simulations and continuum-based theoretical analysis. We vary the NB diameter from 30 nm to 50 nm and study the coalescence characteristics including the expansion speed of the capillary bridge between two coalescing NBs, the dynamic regime of NB coalescence, the diameter of fully merged NBs, and the temperature variation of NBs during the coalescence process. For all cases, we show the MD simulation results can be well understood by the theoretical models developed in this work. Due to the large Laplace pressure in the model NBs, the diameter ratio of fully merged NBs to their daughter NBs is , which explains the recent experimental result showing that the size of NBs in water is distributed discretely with a uniform increment factor of &nbsp;[Ma et al. J. Phys. Chem. B 124, 5067 (2020)]. The expansion of gas inside the coalescing NBs and the heat transfer between the gas NB and surrounding liquid leads to fluctuations of gas temperature during coalescence. From the theoretical analysis, we find the coalescence dynamics of NBs is in the crossover regime where neither viscous stress nor inertial stress in the surrounding liquid dominates even when the viscous stress is more than ten times higher than inertial stress. In the range of Ohnesorge number from 0.33 to 0.82, we show the scaling exponent for the capillary bridge radius vs. time at late times of NB coalescence is around 0.75±0.05, which is considerably higher than 0.5 in the viscous-dominated regime.&nbsp;We conclude the coalescence of N2 NBs in water is in a crossover dynamic regime where neither viscous stress nor inertial stress in the surrounding liquid dominates.

Coalescence Characteristics of Bulk Nanobubbles in Water: A Molecular Dynamics Study Coupled With Theoretical Analysis

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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